Once blacks began to move to the big cities of the north, they moved to the one that were cheapest to go to by train, usually New York, and Philly, as the blacks began to move in, the predominant whites of Harlem were being intimidated by the blacks, and they began to move out. and this caused a huge drop in property prices or also known as a real estate slump, which in turn caused blacks to move in in large numbers, by the 1920's 200,000 African Americans lived in a neighborhood that had been virtually all-white 15 years before. (digitalhistory.uh.edu)